I understand why most Bosses wouldn't have times here, but what about Black Bowser? He can be refought at any time if I remember correctly.

Would be odd for just one boss to have an IL. I don't see a point unless it's every chapter boss or nothing. Even then, we'd have to come up with timing rules, you'd have to import save files just to do runs - all too much work for categories that would be dead or empty.